1. Field of the Invention
The present invention relates to methods for producing a 2-hydroxyethyl methacrylate polymer, a 2-hydroxyethyl methacrylate hydrogel and a 2-hydroxyethyl methacrylate water-containing soft contact lens, which exhibit improved heat resistance and transparency.
2. Related Art
Hydrogels are used as medical materials, in particular, as a material for water-containing soft contact lenses.
Because hydrogels contain water, microorganisms are likely to propagate in medical devices made of hydrogels. Therefore, such products are usually sterilized by pressurization and heating, i.e., by autoclaving before their shipment. These products also must be sterilized upon use. Heat disinfection and chemical disinfection are usually used, and heat disinfection is more effective and hence used more commonly. Therefore, hydrogels used for medical devices such as water-containing soft contact lenses should exhibit high durability against sterilization and disinfection by heating.
As hydrogels for medical devices, those mainly composed of 2-hydroxyethyl methacrylate (abbreviated as HEMA hereinafter) are widely used (see Japanese Patent Publication No. Sho 41-12629 (JP-B-12629/66) and Japanese Patent Publication No. Sho 47-3735 (JP-B-3735/72)).
HEMA mentioned above was originally supplied as a monomer mixture containing methacrylic acid, ethylene glycol dimethacrylate, alkylene glycol monomethacrylate or dimethacrylates and the like as impurities. However, with recent general improvement of the purity of industrial products, HEMA with 3% or less of the above impurities has become commercially available. Further, HEMA of a high purity of 99% or higher is also commercially available at present.
HEMA of such a high purity is useful as feed stock for production of water-containing soft contact lenses by cast polymerization, which requires precise control of polymerization conditions. Water-containing soft contact lenses of stable quality can be obtained by the above cast polymerization.
However, as a result of present inventors' research, it was found that, when sterilization by autoclave and heat sterilization were repeated, surface conditions of water-containing soft contact lenses produced from HEMA of such high purity as mentioned above are degraded, so that they suffer from problems such as light scattering and white clouding when observed on the surface by the naked eye surface observation by naked eye or stereoscopic microscope.
Further, when the transcription of a molding surface is insufficient in cast polymerization which utilizes molds with molding surfaces forming a cavity of a contact lens shape, white clouding may be observed on the surfaces of water-containing contact lenses produced by imparting water into the obtained polymer because of light scattering caused by ununiformity of the surfaces. As a result of the present inventors' research, it was also found that such a phenomena becomes particularly remarkable when the above-mentioned high purity HEMA is used.
As a method for improving heat resistance of the HEMA hydrogel, it is known to copolymerize a crosslinking agent or a hydrophobic component with HEMA (see Japanese Patent Unexamined Publication No. Hei 4-335007 (JP-A-335007/92)). However, HEMA copolymerized with a crosslinking agent or hydrophobic component may disadvantageously exhibit inferior configuration recovery because of increased elastic coefficient, increased brittleness of the hydrogel, decreased water content and the like.
In order to improve the transcription of the molded surface, it is also known to add a solvent or water to monomers containing HEMA upon polymerization so that polymerization shrinkage is decreased (see Japanese Patent Unexamined Publication No. Hei 4-110311 (JP-A-110311/92)). In this method, a diluent replaceable with water such as a boric acid ester of dihydric alcohol is added to a monomer mixture upon direct molding of contact lenses. The use of the diluent enables a decrease in mold opening power and surface defects of the molded articles. However, in such a method, the obtained polymer is a gel containing a solvent or water which is a component of the diluent. This means that a soft and weak gel product should be handled in a process where the gel released from the mold is equilibrated with a solution in which contact lenses are used, e.g., physiological saline. Operability of such a process would be bad and handling of such a gel would be troublesome.
Therefore, an object of the present invention is to provide a water-containing soft contact produced by using HEMA of high purity, which is excellent in heat resistance.
Another object of the present invention is to provide a polymer and hydrogel, which are produced by using HEMA of high purity and can be an intermediate for the above water-containing soft contact lens.
A further object of the present invention is to provide a method for producing water-containing soft contact lenses exhibiting excellent heat resistance and methods for producing polymers and hydrogels which can be an intermediate of the above water-containing soft contact lenses, where HEMA of high purity is used and polymerization conditions can be controlled precisely.
A still further object of the present invention is to provide a method for producing water-containing soft contact lenses exhibiting excellent transparency and heat resistance by cast polymerization, and methods for producing polymers and hydrogels which can be an intermediate of the above water-containing soft contact lenses, where HEMA of high purity is used and polymerization conditions can be controlled precisely.